The invention relates to the production of carbon nanomaterials including fullerenes in sooting flames, and in particular to burners, combustion apparatus, and methods for carbon nanomaterial production. More specifically the invention relates to combustion apparatus and method of combustion for production of carbon nanomaterials in which a liquid hydrocarbon feedstock is introduced into a flame in the form of droplets, e.g., by spraying.
The term “carbon nanomaterials” is used generally herein to refer to any substantially carbon material containing six-membered rings that exhibits curving of the graphite planes, generally by including five-membered rings amongst the hexagons formed by the positions of the carbon atoms, and has at least one dimension on the order of nanometers. Examples of carbon nanomaterials include, but are not limited to, fullerenes, single-walled carbon nanotubes (SWNTs), multiple-walled carbon nanotubes (MWNTs), nanotubules, and nested carbon structures with dimensions on the order of nanometers. The term “fullerene” is used generally herein to refer to any closed cage carbon compound containing both six- and five-member carbon rings independent of size and is intended to include the abundant lower molecular weight C60 and C70 fullerenes, larger known fullerenes including C74, C76, C78, C80, C84 and higher molecular weight fullerenes C2N where N is 43 or more. The term is intended to include “solvent extractable fullerenes” as that term is understood in the art (generally including the lower molecular weight fullerenes that are soluble in toluene or xylene) and to include higher molecular weight fullerenes that cannot be extracted, including giant fullerenes which can be at least as large as C400. In certain cases, carbon nanomaterials may be isolated from the soot or enriched in the soot. Soot produced during the synthesis of carbon nanomaterials, such as fullerenes, typically contains a mixture of carbon nanomaterials which is a source for further purification or enrichment of carbon nanomaterials or which may itself exhibit desired properties of carbon nanomaterials and be useful as an addition to convey those properties. The methods and apparatus of this invention can be used to prepare condensables including soot that contain carbon nanomaterials and can be used in particular to prepare fullerenes and fullerenic soot. The apparatus of this invention is typically run at sub-ambient pressures and as such is particularly useful for the synthesis of carbon nanomaterials that are preferably formed under sub-ambient pressures, e.g., fullerenes and fullerenic soot.
Carbon nanomaterials have been proposed for a variety of application. Fullerenes and fullerenic soot can be employed, for example, as additives to electron- and photo-resists for semiconductor processing; in proton-conducting membranes for fuel cells, as optical limiting materials, in lithium battery anodes; as active elements in organic transistors; as pigments in cosmetics; as antioxidants; and as therapeutics, e.g., as anti-viral agents.
While the art recognizes significant potential for commercial application of carbon nanomaterials, and particularly for fullerenes, SWNTs and MWNTs, the high costs of synthesis of and the difficulties in obtaining these materials in the large amounts necessary for developing these applications has been a major impediment to practical application of these materials. There is a significant need in the art for improved methods and apparatus for making carbon nanomaterials, including fullerenes, in sooting flames that can lower the cost of production and provides these materials in sufficient quantities for practical application.
Sooting flames are currently the most cost-effective way to produce carbon nanomaterials at high production rates (preferably greater than about 100 g/day). This invention relates to improved methods for synthesizing carbon nanomaterials by combustion methods employing sooting flames.
It is known in the art that special fuels and combustion conditions are required for production of substantial amounts of fullerenes and other carbon nanomaterials relative to soot. During normal or industrial combustion the formation of fullerenes relative to soot is so low that these materials can only be detected with the most sensitive analytical techniques (K.-H. Homann, Angew. Chem. Int. Ed. 1998, 37, 2434–2451). Burner design is one of the variables that must be optimized to provide efficiency and acceptable rate of fullerene production (A. A. Bogdanov et al, Technical Physics, Vol. 45, No. 5, 2000, pp. 521–527). This invention relates to improved burner designs particularly adapted for use with lower volatility aromatic hydrocarbon feedstocks which facilitate the production of enhanced levels of carbon nanomaterials.
Fullerenes have been synthesized in premixed flames stabilized on a water-cooled flat metal plate (cooled burner) (Howard et al., U.S. Pat. No. 5,273,729). The device reported employs a burner having a porous burner plate forming the outlet for gases from the burner. The burner plate is water-cooled to prevent the ignition of the fuel-oxidizer mixture in the burner and to stabilize the flame. As the gas velocity through the burner is increased, the flame front tends to move away from the burner surface which can result in flame instability (i.e., the flame can be extinguished). Cooling of the burner plate promotes heat loss from the burner surface causing the flame front to move back towards the burner surface.
However, cooling of the burner surface promotes deposit formation on that surface which can result in irregularities in gas flow, which lead to inhomogeneities in the flame, and can adversely affect the material production yield and homogeneity. When the burner surface becomes coated with deposits, the synthetic process must be stopped to clean the burner. Efficiency of synthesis decreases and the costs of synthesis increase when processing must be interrupted frequently to clean the burners.
U.S. patent application Ser. No. 10/098,829, filed Mar. 15, 2002, reports the use of a burner for production of carbon nanomaterials in which the burner plate does not require cooling to maintain flame stability. This burner can operate at higher temperatures decreasing the rate of buildup of deposits on the burner plate so that the burner needs to be cleaned less often. Further, it is more efficient to operate an uncooled burner, which can heat gas flows, raising flame temperature without significant heat loss due to cooling the burner plate. The burner described in this patent application employs porous refractory material as the burner plate. In addition, the burner plenum is optionally provided with temperature control, e.g. a liquid jacket for heating or cooling, to facilitate burner operation.
Another advantage of avoiding cooling of the burner surface is the ability to introduce lower vapor pressure additives and or fuels or feedstocks into the flame as gases while avoiding condensation in the burner. One example of such additives are high-boiling (lower volatility) PAH rich feedstocks that serve as cost-effective, high-yield feeds for fullerene production, such as those described in U.S patent application Ser. No. 10/099,095, filed Mar. 15, 2002. Another example is catalysts that sublime at elevated temperatures, easing their incorporation into the feed stream.
High temperature surface burners for use in other applications such as industrial furnaces are known. For example, Abe et al., U.S. Pat. No. 4,673,349 describe a high temperature surface combustion burner which uses a burner plate made of a porous ceramic body. In both embodiments of the invention reported, the porous ceramic body contains throughholes. U.S. Pat. No. 4,889,481 to Morris et al. reports a dual structure porous ceramic burner plate for use as an infrared burner. U.S. Pat. No. 5,470,222 to Holowczack et al. reports a high emissivity porous ceramic flame holder for use in a heating unit.
U.S. Pat. No. 5,876,684 (Withers, J. C. and R. O. Loutfy) reports a process for feeding graphite powder into a flame created on a water-cooled burner. U.S. Pat. No. 5,985,232 (Howard et al) speculates that other types of flames and other types of burners could be used to produce fullerenes, but gives no examples of such burners, no procedures for burner operation and flame generation, and no results from any burner other than a premixed water-cooled flat plate burner.
Carbon black can be produced by spraying a liquid hydrocarbon into a natural gas flame. Carbon black reactors such as those described in U.S. Pat. Nos. 4,228,131; 4,250,145; 5,069,882; 5,188,806; 5,254,325; 5,264,199; 5,651,945; 6,096,284; 6,099,818 are operated at ambient or near-ambient pressure. In contrast, carbon nanomaterials synthesis is run at sub-ambient pressures. Carbon black reactors have been found to make only ppm-level quantities of fullerenes. Typically, in carbon black reactors droplets of hydrocarbons and hot combustion gases can be mixed in a turbulence-inducing venturi in the gas flow path. This method of mixing is not easily available for the production of fullerenes and fullerenic soot because it is difficult to create turbulence at the low pressures required for fullerene formation in combustion systems. Only trace (ppm) levels of fullerenes have been identified in the products of carbon black processes.